General Background
Coherent optical radiation, or laser light, is used extensively in communications, medicine, research, imaging, and in many other areas of technology. In such applications, the laser radiation may be used directly or may be used as an intermediate pump source for purposes of promoting amplification or subsequent laser action. When an application's power requirements are small, on the order of several milliwatts more or less, and beam quality is not an overriding concern, laser diodes have been extensively employed because they are directly modulatable and of convenient size and acceptable beam quality. Where modest power is needed, on the order of a watt or so with superior beam quality, such as a diffraction-limited spot, optical fiber lasers with solid state pump sources have been used. For higher power applications where several watts may be needed, as in certain printing applications, use has been made of laser diode arrays as pump sources coupled to special fiber gain structures. For even higher power requirements, ten watts or more, high power laser diode arrays, whose cavities emit a few modes, may be coupled to such specialty gain fibers. However, care must be taken to assure efficient coupling power if maximum power benefit is to be achieved. Since single-mode cores are small, 10 .mu.m or less, and typical materials limit the size of fiber numerical apertures (NA), it is virtually impossible to efficiently couple multimode laser array energy directly into single-mode gain cores via end fire coupling techniques.
As mentioned above, high-power laser light can be obtained by combining the outputs from the emitting cavities of laser diode arrays. However, combining the separate outputs in such a manner that brightness is conserved and power losses are kept to a minimum can be difficult to achieve.
The difficulty results from the fact that the aggregate output of a multiple laser source is not configured for efficient coupling into an optical fiber due to a mismatch between geometric properties and numerical apertures of the source and the receiving fiber. A typical multiple laser source may be a laser bar 10, as shown in FIG. 1. Optical radiation is emitted from a light-emitting face 11 along which are positioned a plurality of laser cavities. In the example provided, light-emitting face 11 includes a laser diode 12 positioned at an array interval spacing 16 from an adjacent laser diode 13. Laser diode 12 typically has an emitting facet output in the shape of a long, narrow rectangle and is oriented with its long dimension parallel to a laser bar axis 14.
For the purpose of illustration, a set of laser-beam uvw-coordinates 19 is used to describe the propagation characteristics of the beams of radiation emitted from the laser diodes. The orientation of the w-axis is perpendicular to light-emitting face 11 and coincident with the direction of propagation of the beams of radiation. Coordinate set 19 "travels" with each beam, rotating about the w-axis as the beam is rotated, and changing direction as the beam's direction of propagation is changed. Laser diode 12 emits a laser beam 20 and laser diode 13 emits a laser beam 21 and so on.
The radiation distributions of the emitted laser beams 20 and 21 are represented by ellipses to indicate that they each have a v-component parallel to laser bar axis 14 and a u-component perpendicular to laser bar axis 14. A more quantitative representation of the laser beam divergence is provided in the graphical illustration of FIG. 2, which shows that each laser beam diverges at a larger angle .theta..sub.u in the u-direction than the angle of divergence in the v-direction .theta..sub.v, as the laser beam propagates in the w-direction. NA values, measured to include 95% of the optical power, are typically 0.70 to 1.75 (40.degree. to 100.degree.) for NA.sub.u and 0.14 to 0.35 (8.degree. to 20.degree.) for NA.sub.v. Before such laser beams can be guided into an optical fiber, a coupling device is needed to reformat the radiation into a more suitable configuration that is more compatible with the geometry and the NA of the fiber.
A measure of coupling mismatch between two optical components can be provided by a quantitative comparison of the "etendu" values for the two components. The etendu of a component is defined as the mathematical product of the angular extent and the spatial extent of the radiation entering or emitting from that component: EQU etendu .DELTA. [angular extent].times.[spatial extent]
To illustrate, assume laser bar 10 to have a linear array of twenty laser diodes on a face 1.00 cm long by 0.1 mm wide. If laser diodes 12 and 13 are one .mu.m in the u-direction and 175 .mu.m in the v-direction, with an array interval spacing 16 of 485 .mu.m center-to-center, NA.sub.u 25 is approximately 0.55 (31.5.degree.) and NA.sub.v 27 is approximately 0.12 (6.9.degree.) for a wavelength of 1.06 .mu.m, as indicated in FIG. 1.
For laser diode 12, the u-component etendu value becomes 1 .mu.m.times.0.55 NA, or 0.55 .mu.m-NA, and the v-component etendu value is 175 .mu.m.times.0.12 NA, or 21 .mu.m-NA. For laser bar 10, the u-component etendu is also 0.55 .mu.m-NA. The v-component etendu for laser bar 10 is 1,200 .mu.m-NA, which is more than two thousand times as great as the u-component etendu. In comparison, a typical optical fiber core with an NA of 0.1 and diameter of 7.5 .mu.m has an etendu of 0.75 .mu.m-NA. Direct coupling of the laser array into an optical fiber would consequently be extremely inefficient.
A mismatch of this magnitude cannot be corrected solely by the use of anamorphic imaging systems even though they have different spatial magnification in the two orthogonal directions. Any practical imaging system which decreases the etendu mismatch between a laser diode array and an optical fiber must perform more complicated reformatting tasks including rotating each emitted diode beam by 90.degree. before optical corrections to the beam are made by the imaging system.
Hence, because of incompatible beam configuration, coupled with inherent practical limits on optical fiber numerical apertures (i.e., solid acceptance cones), it remains difficult, if not impossible, to efficiently couple these higher power sources into cores, especially single-mode cores which would require a single-mode pump for efficient coupling.
However, Snitzer et al. disclosed an elegant solution to this problem in U.S. Pat. No. 4,318,079, and provided a significant improvement over an earlier approach by Maurer, as described in his U.S. Pat. No. 3,808,549. In the Snitzer et al. scheme, now referred to as "cladding pumping", a single-mode core containing the active ion is surrounded by an undoped inner multimode cladding of lower index than that of the core and is of a special geometry for efficient pumping. This, in turn, is surrounded by an outer cladding of yet lower index of refraction. Pump light is launched into the inner cladding and is confined by total internal reflection at the interface between claddings to propagate down the inner cladding, which is a core-like structure with respect to the outer cladding. The inner cladding, being multimode, is obviously physically larger than the core and therefore presents a better target, and the numerical aperture, being a function of the indices of the inner and outer claddings, is made as large as possible to more efficiently receive pump power. As pump power propagates down the inner cladding, it is progressively absorbed by the core to provide the population inversion necessary for gain and subsequent laser action with suitable feedback. This scheme is a hybrid having the character of both longitudinal and transverse pumping, and has the great merit of efficiently coupling available pump power from an incoherent source to a single-mode core to provide single-mode output. Inner cladding geometries that have been found efficacious include elongated slab configurations, like the rectangle, and a configuration in which a core is eccentrically located inside of the inner cladding.
It is therefore an objective of the present invention to provide a high-power optical fiber device which combines the outputs of multiple laser sources into a single high-power beam of optical radiation.
It is a further object of the invention to provide such an optical fiber device in which the single output beam can be efficiently coupled into an optical fiber core.
It is a further object of the invention to provide such an optical fiber device in which the inner cladding cross-sectional shape can be efficiently matched to different input beam characteristics.
It is another object of the invention to provide such an optical fiber device in which the optical fiber provides a uniform distribution of radiation modes within the inner cladding.
It is yet another object of the invention to provide such an optical fiber device in which the fiber radiation coupling efficiency is not a function of the location of the fiber core.
Other objects of the invention will, in part, appear hereinafter and will, in part, be apparent when the following detailed description is read in connection with the drawings.